What is hot and cold?


We can feel heat and cold but we can’t really see it. What is actually happening and why do we prefer a specific temperature?

In: Physics



Heat is energy in the form of molecules moving around in sort of random chaotic vibrations. This is easiest to imagine for a gas, where the molecules travel long distances in straight lines between collisions. It is (for reasons based in statistics) the ‘final form’ of all mechanical energy. A moving object will tend to convert its motion energy into heat.

For those same statistical reasons, heat spreads out evenly. If you have heavy molecules and light molecules, the light molecules will tend to move faster in order to have the same energy (on average) as the heavier ones. The electrons and atoms and such all get their split of the heat energy.

Heat indirectly results in temperature. Adding heat to an object usually raises its temperature, although some processes (such as melting or boiling) absorb heat without raising the temperature.

Many chemical processes, like the aforementioned melting and boiling, act differently with different amounts of molecular motion. That’s what melting is, really; a material acting differently because its molecules have more energy.

Humans have many chemical processes in our bodies. These processes happen at different speeds and in different ways – especially in the deeper core of our bodies it is critical that each of these processes operate consistently in the same way. To allow this, we maintain a specific internal temperature. To help maintain an internal temperature, our bodies change many things, but that’s its own eli5.

In its most basic form, heat is motion. When things get hot, it’s because the atoms that make up those things are excited and caused to move.

Cold is fun because it definitely exists but it also doesn’t. A thing is cold when it’s atoms are moving less.

When excited atoms from something interact with, say your hand, they excite the atoms in your hand and begin to increase their motion. This would feel warm/hot until the motion equalizes.

If the atoms in your hand are moving faster than those in the object it will feel cool/cold and the atoms in your hand will excite the atoms in the object until the motion of the atoms in both is equal.

Fun fact this is why the universe is doomed. Feel free to correct/add I’m surely no expert.

Temperature is pretty much quantifying the average kinetic energy of molecules/atoms in a substance. So the hotter the substance, the faster its molecules are moving around. Some molecules are fast/slow but the average will be higher.

When that substance comes into contact with something, the molecules of the two substances will start colliding. Whichever molecule is moving faster (hotter substance) will transfer energy to the slower one (colder substance) gradually as the fast molecules hit slow ones.

As far as preferring a temperature, it just has to do with how your cells function. I’m not sure on the specifics, but if cells get too hot or cold the proteins within them can get screwed up, or regulation of nutrients can go haywire.

In definition temperature is the speed at which matter is moving. But an animals ability to perceive and react to temp is different. The way an organism regulates its body temp is a direct result of the environment in which it naturally evolved. Humans are primates which exist in tropical/subtropical environments (except for the Japanese macaque). Humans of course are an anomaly because we can live anywhere on the planet because of our intelligence. But the fact remains we cannot change what we evolved from and we regulate our body temp best in mild temperatures with moderate humidity like the climate of Eastern Africa during the Pleistocene.

The human body is also better at cooling down than retaining heat. We have more sweat glands per sq. inch than any mammal and we are essentially hairless. This is why the invention of clothing and the mastery of fire was so essential to our success.